Field of the Invention
The invention concerns to the operation of a medical imaging examination apparatus having multiple subsystems, a corresponding medical imaging examination apparatus, and an associated electronically readable data carrier encoded with programming instructions for implementing such a method.
Description of the Prior Art
Medical imaging examination apparatuses such as magnetic resonance apparatuses and computed tomography apparatuses are complex systems with a large number of technical subsystems. These include, in a magnetic resonance apparatus, a basic field magnet system, a gradient system, a shim system and a radio frequency transmission system as well as a radio frequency receiving system.
In order to generate images or spectroscopic data from an examination object with a magnetic resonance apparatus, the examination object is positioned in the scanner in a strong homogeneous basic magnetic field, also known as the B0 field, generated by the basic field magnet system with a field strength of 0.2 Tesla to 7 Tesla or more, so that the nuclear spins in the object align along the basic magnetic field direction. In order to trigger nuclear spin resonance, radio frequency excitation signals (RF pulses) are radiated into the examination object with suitable antennas of the radio frequency transmission system, so that the nuclear spin of particular atoms stimulated to resonance by this radio frequency field are tilted through a particular flip angle relative to the magnetic field lines of the basic magnetic field. The nuclear spin resonance that is triggered, i.e. the radio frequency signals (also “magnetic resonance signals”) emitted during the precession of the nuclear spin are detected by the radio frequency receiving system, typically digitized, and normally stored as complex number values (if a spatial reference is given) in a “k-space matrix” as “k-space data”. For example, in single-voxel spectroscopy scans (without spatial reference), the digitized data are stored as complex time signals, also known as “FID data”. On the basis of the k-space data or FID data, MR images can be reconstructed or spectroscopic data can be determined. For spatial encoding of the scan data, rapidly switched magnetic gradient fields are overlaid on the basic magnetic field by the gradient system. The shim system is intended to homogenize the magnetic fields.
All these technical modules must be suitably operated in a coordinated way by a control system. The adjustment and switching of the individual subsystems necessary for a particular imaging process must be undertaken by the control system at the right time point in each case. Typically, the volume to be imaged within an imaging sequence is recorded in subvolumes, for example, in 2-D imaging, in multiple slices or, in 3-D imaging, in multiple “slabs”. The subvolumes recorded in this way are then assembled into an overall volume. A further definition of subvolumes can be given as “regions of interest” (ROI) or “volumes of interest” (VOI) defined, for example, by the operator. Furthermore, in magnetic resonance systems, additional subvolumes arise when determining local saturation regions or local preparation or labeling pulses.
As mentioned above, sequence control data are transmitted to the control device for coordinated control, typically based on a “scan protocol”. These sequence control data define different functional subsequences of a complete scan sequence. In a magnetic resonance recording, for example, a first subsequence may be a pulse sequence in order to achieve a saturation locally in a particular region. Further subsequences can contain, for example, particular preparation pulses and yet further subsequences serve for successive excitation and for receiving the magnetic resonance signals in different slices or slabs.
Typical methods based on MR technology, such as tomographic imaging (MRT—magnetic resonance tomography) or spectroscopy (MRS—magnetic resonance spectroscopy) require “benign” ambient physical conditions in order to ensure the best possible quality in the data recorded. For example, this relates to the spatial homogeneity, temporal stability and the absolute accuracy of the relevant magnetic fields and radio frequency fields, that is, the main magnetic field (B0) and the gradient and radio frequency fields (B1).
Conventionally, deviations from ideal ambient conditions can at least partially be compensated for, for example, by system-specific settings known as “tune-ups”, in particular with regard to eddy current-induced dynamic field disruptions or gradient sensitivities or by examination object-specific settings, particularly in relation to susceptibility-related static field disruptions or spatial variations of the radio frequency field. However, the compensation settings specified before the beginning of a scan conventionally remain valid throughout the entire scan (“static” adjustment).
For spatially variable ambient conditions that cannot be entirely compensated, this entails a compromise for data quality.
De Graaf et al. describe in “Dynamic Shim Updating (DSU) for Multi-Slice Signal Acquisition”, Proc. Intl. Soc. Mag. Reson. Med. 10, p. 536, 2002, a rudimentary form of a dynamic adjustment of the shim currents of the field coils for the B0 shim in functional multi-slice MR imaging. For this purpose, a firm field deteimination sequence is created for determining spatial field changes of first or higher orders which must be exactly matched to the corresponding parameters (e.g. slice positions and orientations) of the desired imaging sequence. The field determination sequence records the data necessary for field determination and analyzes them in order to calculate optimized shim currents (of first or higher order) therefrom for each slice to be scanned with the imaging sequence. Subsequently, the imaging sequence is started with the optimized shim currents. The user needs to watch very closely for consistency between the imaging sequence and the field determination sequence since, otherwise, inconsistencies can lead to a worsening of the image quality. Therefore, for each imaging sequence and each change of such a sequence, a new field determination sequence must be created and carried out before the scan with the imaging sequence. These methods are therefore very complex and difficult for the user to combine with other, for example static, adjustments since interactions between different parameters cannot be taken into account or only to a limited extent. If statically adjusted parameters are changed, this can have effects on the optimum dynamic settings of the shim currents and a new field determination sequence and calculation of the optimized shim currents would have to be carried out. Furthermore, the optimization is restricted to the slices of the imaging sequence. Smaller volumes, for example, regional saturation volumes are not taken into account here.
In DE 10 2009 020 661 B4 also, a method is described with which parameters of a scan sequence, for example, within magnetic resonance technology can be adapted at the run time of the scan sequence. Furthermore, it is described therein that different functional subsequences are typically associated with different effective volumes. I.e. for each subsequence, a different subvolume of the overall scan volume is relevant. Due to the determination of the parameters at run time, it can however occur that, in the time available which is limited due to the already running scan sequence, no useful parameters can be determined. In this event, either the scan as a whole can be terminated or sub-optimum, static parameters can be utilized.
Furthermore, techniques are already known that detect changes in the ambient conditions and take account of the effects of the changes on the scan data either retrospectively, i.e. correcting the effects of the changes subsequently, or prospectively, for example, by tracking parameters of the data recording. For example, echo planar imaging (EPI) is an intrinsically very sensitive method that, due to a very small pixel bandwidth in the phase encoding direction (of the order of 10 Hz/pixel), reacts to even small basic magnetic field changes with severe image displacements. In this regard, it is known from DE 10330926 A1 to measure the basic magnetic field during the recording of an EPI time series and then to reverse the effect (the displacement) on the image data retrospectively. From the article by Benner et al. “Real-Time RF Pulse Adjustment for B0 Drift Correction”, Magn. Reson. Med. 56:206 (2006), a prospective method is known that also measures the basic magnetic field change and adjusts the RF center frequency of the MR scan accordingly. Prospective methods are usually preferable because retrospective methods can correct only a part of the effects relevant to the image, e.g. a constant efficiency of the chemically-selective fat suppression can be achieved only prospectively.
However, the known techniques permit only a global reaction to changes in the ambient conditions and are therefore forced to make local compromises.